Conventional semiconductor devices generally include a semiconductor substrate, such as a silicon substrate, and a plurality of sequentially formed dielectric interlayers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Copper and copper-alloys have recently received considerable attention as interconnect materials because of their superior electro-migration and low resistivity characteristics. The interconnects are usually formed by filling copper in features or cavities etched into the dielectric layers by a metallization process. The preferred method of copper metallization is electroplating. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in sequential layers can be electrically connected using vias.
In a typical process, first an insulating layer is formed on the semiconductor substrate. Patterning and etching processes are performed to form features or cavities such as trenches and vias in the insulating layer. Then, a barrier/glue layer and a seed layer are deposited over the patterned surface and a conductor such as copper is electroplated to fill all the features. However, the plating process, in addition to filling the features with copper, also deposits excess copper over the top surface of the substrate. This excess copper is called an “overburden” and needs to be removed during a subsequent process step. In standard plating processes this overburden copper has a large topography since the Electrochemical Deposition (ECD) process coats large features on the wafer in a conformal manner. Conventionally, after the copper plating, Chemical Mechanical Polishing (CMP) process is employed to first globally planarize this topographic surface and then to reduce the thickness of the overburden copper layer down to the level of the surface of the barrier layer, which is also later removed leaving conductors only in the cavities.
During the copper electrodeposition process, specially formulated plating solutions or electrolytes are used. An exemplary electrolyte contains water, acid (such as sulfuric acid), ionic species of copper, chloride ions and certain additives, which affect the properties and the plating behavior of the deposited material. Typical electroplating baths contain at least two of the three types of commercially available additives such as accelerators, suppressors and levelers.
Electroplating solutions such as the commonly used copper sulfate solutions employed for copper film deposition naturally contain dissolved air since they are in contact with air. While in use in plating tools these electrolytes may further get saturated with air since they are often cycled between the plating cell and an electrolyte tank. After being used in the plating cell for plating copper onto the workpiece surface, electrolyte is recycled by directing it back to the main tank, and after filtration and chemical composition adjustment, it is pumped back into the plating cell. Such recycling minimizes electrolyte waste, however, at the same time it increases air dissolution into the electrolyte. In some prior art approaches, a nitrogen blanket has been used over the electrolyte tank and other parts of the system to minimize exposure of electrolyte surface to air. There have also been methods that involved bubbling nitrogen through the electrolyte to reduce oxygen content in the solution. Such efforts may understandably reduce the concentration of dissolved oxygen in the electrolyte, however they do not reduce the total dissolved gas content of the solution. In fact, such approaches are expected to enhance the dissolution of the blanket gas, such as nitrogen, in the electrolyte. In other words, gas content in the electrolyte would still be high, although its chemical composition would be different, i.e., there would be more nitrogen gas and less oxygen.
Dissolved gas in plating electrolytes creates several problems. First of all, dissolved gas in any liquid causes initiation and growth of bubbles on surfaces touching the liquid. For example, when a workpiece, such as a semiconductor wafer is immersed into a copper-plating electrolyte with dissolved air in it, micro-bubbles of gas often spontaneously initiate on the surface of the wafer. Initiation and growth rate of such micro-bubbles are expected to be a function of the degree of saturation of the liquid by the gas, the temperature of the electrolyte and the pressure. Highly agitated electrolytes in the presence of a gas, such as air, get highly saturated with air and therefore bubbles form on surfaces touching such electrolytes very easily. Similarly, higher temperatures would promote growth of bubbles faster. Electrolytes pumped from high pressure zones to lower pressure zones would have the dissolved gas more unstable in the low pressure zone, i.e., bubble formation would be promoted in zones where fluid pressure is suddenly reduced (such as after a flow restricting filter). FIGS. 1A, 1B, 1C and 1D schematically depict one of the consequences of such bubble formation.
In FIG. 1A, a silicon substrate 10 is shown with sub-micron size features 12 such as trenches etched into an insulating layer 14 on its surface. The etched features 12 and the top surface 13 are lined with a barrier layer 16 and a copper seed layer 17. As illustrated in FIG. 1B, the substrate 10 is then placed into a copper plating solution 18 for copper deposition. FIG. 1B represents the instant when the substrate 10 is immersed into the plating solution 18, which is saturated with air or has a large concentration of dissolved gas. The plating is initiated by applying a potential between the conductive substrate surface (barrier layer 16 and/or seed layer 17) and an electrode (not shown) in the plating solution 18. Bubbles 20 represent the micro-bubbles that may initiate on the surface of the seed layer as soon as the wafer is placed in the solution. These bubbles may be micron or sub-micron in size and, they may be within the features 12, on the seed layer portion covering the top surface 13 or at the comers 21. As the plating continues, bubbles 20 retard material deposition onto the locations that they are attached and give rise to defects such as voids as depicted in FIG. 1C. Also shown in FIG. 1C is the possibility of having new bubbles 22 nucleate on surface 24 of the copper layer 26, which is being deposited. FIG. 1D shows the substrate 10 after the copper deposition step is finished. As can be seen in FIG. 1D, various defects 28 are created by the bubbles on the substrate surface either during the initial or later stages of the electrodeposition process. These defects, after the CMP and other process steps employed to fabricate the interconnect structure, cause reliability problems such as poor stress migration and poor electromigration. It should be noted that similar problems with bubbles are present for deposition of copper layers by the electroless deposition techniques.
Formation of bubbles of gas on surfaces placed in plating electrolytes can cause other problems also. Even if bubbles are not formed on the wafer surface, they can form on other surfaces of the plating cell and then migrate to the wafer surface, giving rise to defects already described. In certain wet processing techniques, (such as electrochemical mechanical deposition and electrochemical mechanical polishing) there are pad structures or workpiece surface influencing device (WSID) structures proximate to the wafer surface. These pad structures are used to sweep the wafer surface during the electrochemical mechanical process to planarize or polish the wafer surface. The surfaces of all these structures, which are immersed in the process solutions and placed close to the wafer surface, are also possible sites for bubbles to initiate, grow and eventually migrate to the wafer surface causing defects.